Shelf



FIG. 1 is a perspective view of a shelf, showing my new design, the broken lines are for environmental purposes only and form no part of the claimed design;

FIG. 2 is a top plan view thereof;

FIG. 3 is a front plan view thereof;

FIG. 4 is a lower plan view thereof;

FIG. 5 is a rear plan view thereof; and,

FIG. 6 is an end view thereof. 

The ornamental design for a shelf, as shown and described. 